I am getting riser cables so that I dont run my gpus on the motherboard.  I am using a cooler master case, a cheaper one than this image but this will serve nicely to explain.

I have placed the case in a sleeping position and not standing as it is shown here. I now have 12 inch risers for my 2 gpus (R9 280X). Until I build something custom to hold the gpus, I was thinking of placing a plank on the case and placing the gpus flat with the fans blowing upwards. Will that cause problems or can the gpus be placed like that.